1. Field of the Invention
This invention relates to tube mills of the multi-compartment type for drying and grinding moist material such as cement clinker.
2. Description of the Prior Art
Tube mills of the prior art for drying and grinding moist material, in which the drying is performed by a hot stream of gas or air passing through the mill, are well known. Material usually is fed to both ends of such a mill. Moist coarse material is fed to the end of the mill connected to a drying compartment, and a coarse fraction of the dried (or semi-dried material) which exits the drying compartment is fed to the other end of the mill. The mill has a central outlet such as a central discharge chamber for exiting spent gas and dried, ground material. A portion of the material is entrained in the gas leaving the mill.
The discharge chamber is provided with sieving diaphragms to restrict grinding bodies and oversize material from passing to the discharge chamber. The gas and the material entrained in the gas pass from the central outlet, through openings in the mill shell and into a casing which surrounds the discharge chamber. The gas and entrained material then pass to a separator in which the material is separated into two or more fractions. One fraction constitutes the final product and the remainder constitutes a coarse fraction which is returned to the mill for further grinding.
The sieving diaphragms often give rise to difficulties because they restrict the passage of gas and material to the central discharge chamber. Only a limited amount of gas can thus be forced or drawn through the mill. Treatment of wet raw material also involves increased difficulties since the material sticks to the sieving diaphragm and reduces the sieving area.
A prior art drying and grinding process has been proposed in which all (or part) of the gas stream is discharged from the mill after leaving the drying compartment, while the material proceeds to the succeeding grinding compartment. In this process, however, a substantial part of the material is entrained by the gases and is carried out of the mill with the gases. The gases and entrained material pass to a separator from which at least part of the material coarse fraction is returned to the drying compartment, thereby establishing an undesirable circulation of material. Further, the conventional sieving diaphragm restricts the flow area, thereby limiting the amount of gas which can be drawn or forced through the drying compartment.
I have invented a tube mill, having a separating chamber, which avoids these drawbacks and provides an improved approach for drying and grinding moist material.